Terahertz technology has now received more and more attentions due to its wide applications in various scientific and technical fields. Because of their capability to prohibit the propagation of terahertz irradiation for all directions in their band gaps, three dimensional terahertz photonic crystals (3D-TPCs) with a periodic diamond structure could create specific functionalities. Various techniques had been developed to assembly these complex structures difficult to fabricate by conventional processes, including the particle manipulation assembly and lithographic techniques. However, the particle manipulation assembly is inherently complex and time-consuming, while few materials are suitable for lithographic techniques. Novel approaches should be developed for the fabrication of 3D-TPCs with the easy operation, low cost, and the flexibility on the structure/material design.

The direct-writing technology could create complex 3D structures via a layer-by-layer building process with a broad range of materials at the microscale level, which had demonstrated great potentials for emerging applications. Thus, it could provide a powerful alternative for producing complex 3D-TPCs with various material systems. A recent progress of the direct-writing technology (biomimetic 4D printing) demonstrated that the shape of complex 3D structures could be modulated via a careful design of the ink system, which opened new avenues for the creation of “smart” 3D structures responsive to external motivations. Thus, if 3D-TPCs could be created with tunable terahertz properties under proper external stimulations, novel functions could be integrated to these 3D-TPCs for cutting-edge terahertz technology.

Here, we developed a composite ink system composed of polydimethylsiloxane (PDMS) and barium titanate (BaTiO3) nanoparticles for the creation of mechanically flexible 3D-TPCs with tunable terahertz properties under external force field by the direct-writing technology. By the solid state shear milling process, BaTiO3 nanoparticles of high refractive indices (RI) were well dispersed into PDMS polymer matrix to create these composite inks. In this composite ink system, PDMS serves as the flexible matrix component to provide the reversible mechanical deformation capability for these 3D-TPCs, while BaTiO3 nanoparticles of high refractive indices provide the terahertz response with proper 3D structure design. By varying the content of BaTiO3 nanoparticles to modulate the RI of this composite ink or changing the geometry of these 3D-TPCs, different terahertz properties could be obtained. More interestingly, these 3D-TPCs demonstrate a unique tunable terahertz property under external force field due to their mechanical flexibility from the PDMS matrix of the composite ink. Thus, their terahertz property is responsive to external force fields reversibly, which can find novel applications in terahertz technology and other relative technological applications.